Hi,
This is a very sensitive issue because it depends a lot on server configurations.
This same import process that runs fine in the majority of the servers can run bad in some servers due to the configuration.
Our Demo importer it’s based on the WordPress default Import plugin and it was improved but. The WordPress plugin itself doesn’t guarantee 100% the import success for the same reasons.
In your case it was being generated and internal server error, your server logs may provide additional information but it’s complex to figure out.
I manage to import most part of the content just by running the import plugin a few times. Then were missing just a few things that I exported again from our demo site, and used the default WordPress import functionality and it went fine.
-Rui